Peel and cut the mushrooms into thin slices. Peel and finely chop the onion. In a saucepan, heat the butter and sauté the onion and mushrooms over high heat. When soft, reduce the heat and add salt and pepper. In a bowl, mix the flour with milk until there are no lumps in the mixture and the flour has dissolved completely. Pour the flour mixture in the pan with the mushrooms, add the nutmeg and stir for 5-7 minutes over medium heat until the mixture thickens. Remove the pan from the heat and leave
for 30 minutes until cool. Grate the Edam cheese and cut the ham into small pieces. Combine the mushroom mixture with the cheese, ham and egg yolks.
Prepare the pie according to the instructions and bake for 40-45 minutes at 190°C, on the lowest shelf of the oven to bake the pie's base thoroughly. Let the pie cool before you slice and serve.

For making Pie in a Rectangular Baking Tin:
Butter the bottom and sides of the baking tin. Open the Filo sheets and lay half of them inside the pan, brushing each one with butter. Pour in the filling, fold the remaining part of the sheets inward to seal the filling and cover it with the remaining pastry sheets, brushing each one with butter. Use a knife to fold the overhanging pastry sheets under the rim of the baking tin. Using a sharp knife cut the surface of the pie in portions. Beat the egg and brush the surface.